The long-range objective of this project is to understand the synaptic organization of the cochlear nucleus (CN), where auditory signals first reach the brain. Auditory nerve input reaches the CN in a tonotopically organized manner, where all fibers terminate and synapse with various CN cell types. This primary inputs is not homogeneous. Physiological recordings have indicated that it comprises 3 groups distinguishable by spontaneous discharge rates (SR; Liberman, 78). These groups of high, medium, and low-SR have been correlated with the distribution and morphology of fibers and terminals both peripherally in he cochlea and centrally in the CN (e.g., Liberman, 82; Ryugo and Rouiller, 88). In order to understand the input-output functions of the CN, it is important to determine the ratios of input from the various SR classes to the different CN cell types. Given that morphological analysis. Then morphometric criteria established in this way would identify these synaptic terminals in single electron micrographs--without the need for laborious labeling and serial-section reconstruction procedures. The present proposal will specifically analyze terminals of auditory nerve fibers of known physiology, which have been intracellularly labeled with horseradish peroxidase (HRP) (tissue supplied by M.C. Liberman). Therefore, single fibers can be reconstructed and many terminals throughout the CN are available for morphometric examination. Automated measurements of selected features of these terminals (vesicle size and roundness, vesicle and mitochondrial densities, synaptic length, etc.) will be conducted to rapidly build a quantitative data base of the identifiable morphometric features of the 3 SR groups. Thus far, preliminary results demonstrate that one criterion, the size of synaptic vesicles, apparently differs in terminals of high and low SR. Morphometric features will also be related to the different classes of postsynaptic CN neurons that receive this input. After building up a comprehensive morphological data base from identified and labeled terminals, unlabeled material will be examined on the basis of these criteria in order to determine ratios of input from the physiological classes to various CN cell types.